New XLi3N2 compounds and their hydrides as hydrogen storage materials

ABSTRACT

State-of-the-art electronic structure calculations provide the likelihood of the availability of YLi 3 N 2 , ZrLi 3 N 2 , NbLi 3 N 2 , MoLi 3 N 2 , TcLi 3 N 2 , RuLi 3 N 2 , RhLi 3 N 2 , GeLi 3 N 2 , InLi 3 N 2 , and SnLi 3 N 2  as for reaction with hydrogen under suitable conditions. Such calculations also provide the likelihood of the availability of YLi 3 N 2 H n , ZrLi 3 N 2 H n , NbLi 3 N 2 H n , MoLi 3 N 2 H n , TcLi 3 N 2 H n , RuLi 3 N 2 H n , RhLi 3 N 2 H n , PdLi 3 N 2 H n , AgLi 3 N 2 H n , CdLi 3 N 2 H n , AlLi 3 N 2 H n , GaLi 3 N 2 H n , GeLi 3 N 2 H n , InLi 3 N 2 H n , SnLi 3 N 2 H n , and SbLi 3 N 2 H, (here n is an integer having a value of 1-6) as solid hydrides for the storage of hydrogen. These materials offer utility for hydrogen storage systems.

TECHNICAL FIELD

This invention pertains to compounds useful for solid-state storage of hydrogen. More specifically, this invention pertains to a family of new compounds, XLi₃N₂, which form hydrides, XLi₃N₂H_(n), where X is a 4d transition metal or a neighboring element in the periodic table.

BACKGROUND OF THE INVENTION

Considerable development effort is currently being expended on the development of hydrogen and oxygen consuming fuel cells, and there is also interest in hydrogen burning engines. Such power systems require means for storage of hydrogen fuel which hold hydrogen in a safe form at ambient conditions and which are capable of quickly receiving and releasing hydrogen. In the case of automotive vehicles, fuel storage is required to be on-board the vehicle, and storage of hydrogen gas at high pressure is generally not acceptable for such applications.

These requirements have led to the study and development of solid-state compounds for temporary storage of hydrogen, often as hydrides. For example, sodium alanate, NaAlH₄, can be heated to release hydrogen gas, and a mixture of lithium amide, LiNH₂, and lithium hydride, LiH, can be heated and reacted with the same effect. Despite such progress, however, no known solid-state system currently satisfies targets for on-board vehicular hydrogen storage.

U.S. patent application Ser. No. 11/386,409, titled “XLi₃N₂ Compounds and Their Hydrides as Hydrogen Storage Materials,” by the inventor of this invention and assigned to the assignee of this invention, describes and claims a family of new compounds, XLi₃N₂, which form hydrides, XLi₃N₂H_(n), where X is a 3d transition metal. This invention extends that family to certain additional XLi₃N₂ and XLi₃N₂H_(n) compounds for hydrogen storage.

SUMMARY OF THE INVENTION

FeLi₃N₂ is prepared by reaction of Li₃N melt with elemental iron in a nitrogen atmosphere. It crystallizes in the body-centered Ibam structure (space group 72). Ternary compounds BLi₃N₂, AlLi₃N₂, and GaLi₃N₂ are also known to exist. However, other compounds like XLi₃N₂, where X is any of the 4d transition elements (Y—Cd), and neighboring elements, Ge, In, Sn, and Sb, are unknown. These other ternary nitride compounds would have the same stoichiometry as FeLi₃N₂ and are of interest as hydrogen storage materials where the ternary nitride takes up hydrogen as XLi₃N₂H,.

State-of-the-art computational electronic structure methods, using FeLi₃N₂ as the template compound, indicate that several of these ternary nitrides, XLi₃N₂, are thermodynamically stable. Accordingly, this invention demonstrates the credible likelihood that each of YLi₃N₂, ZrLi₃N₂, NbLi₃N₂, MoLi₃N₂, TcLi₃N₂, RuLi₃N₂, RhLi₃N₂, GeLi₃N₂, InLi₃N₂, and SnLi₃N₂ can be prepared as new materials for storage of hydrogen. The computational methods also show thermodynamic stability of the hydrides YLi₃N₂H_(n), ZrLi₃N₂H_(n), NbLi₃N₂H_(n), MoLi₃N₂H_(n), TcLi₃N₂H_(n), RuLi₃N₂H_(n), RhLi₃N₂H_(n), PdLi₃N₂H_(n), AgLi₃N₂H_(n), CdLi₃N₂H_(n), AlLi₃N₂H_(n), GaLi₃N₂H_(n), GeLi₃N₂H_(n), InLi₃N₂H_(n), SnLi₃N₂H_(n), and SbLi₃N₂H_(n), where n is an integer having a value of 1-6. Accordingly, this invention also provides the likelihood of a hydrogen storage compound for each of the specified ternary nitride compositional formulas.

In some instances, hydrogen may be released from the hydride, XLi₃N₂H_(n) to form the corresponding parent compound, XLi₃N₂. In other instances, release of hydrogen from the hydride also yields other chemical species containing the original metal, lithium and nitrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the experimentally determined structure of FeLi₃N₂ that is the template for computational electronic structure methods showing the thermodynamic stability of other isostructural ternary nitride compounds, XLi₃N₂. In this figure, the large dark-filled circles represent the positions of nitrogen atoms, the speckled circles represent the positions of iron atoms, and the small unfilled circles represent positions of lithium atoms.

FIG. 2 illustrates the calculated structure of FeLi₃N₂H₂ with H atoms occupying the 8j sites in the Ibam structure; it is the most thermodynamically stable hydride of FeLi₃N₂ identified with the computational methodology used here. In this figure, the large dark filled circles represent the positions of nitrogen atoms, the speckled circles represent the positions of iron atoms, the small unfilled circles represent positions of lithium atoms, and the small dark filled circles represent positions of hydrogen atoms.

DESCRIPTION OF PREFERRED EMBODIMENTS

State-of-the-art computational electronic structure methods implementing density functional theory (DFT) have been employed with substantial success to model hydride properties, including the crucial enthalpies of formation. That success encourages the development of strategies for harnessing the calculational tools to guide the discovery of novel hydrides. The approach in this case is to choose a compound having a known crystal structure and calculate enthalpies of formation for isostructural, hypothetical compounds constructed by elemental replacements and the addition of hydrogen to the original lattice.

In this work FeLi₃N₂ is selected as the template compound, and the formation of isostructural XLi₃N₂ phases and their XLi₃N₂H_(n) hydrides with X any of the 4d transition elements (Y—Cd) and neighboring elements Al, Ga, Ge, In, Sn, and Sb is considered. Searching for hydrides comprising a 4d element such as Y or Zr to facilitate H₂ dissociation and lighter elements such as Li to enhance the gravimetric hydrogen density is the strategy.

Aside from FeLi₃N₂, compounds having the XLi₃N₂ stoichiometry are only known for X═Al, Ga, and B. AlLi₃N₂ and GaLi₃N₂ both crystallize in the body-centered cubic |ā3 structure (space group No. 206). BLi₃N₂ forms in at least three structures all distinct from Ibam: (i) a tetragonal P42212 low-temperature phase, (ii) a monoclinic P2₁/c phase often observed at high temperatures, and (iii) a recently identified body-centered tetragonal I4₁/amd phase. X—Li—N hydrides have only been established definitively for X═B. A hydride of BLi₄N₃H₁₀ stoichiometry has been discovered. Other hydrides have been tentatively identified in the B—Li—N—H system.

Crystal Structure of FeLiN₂

FeLi₃N₂ crystallizes in the body-centered orthorhombic Ibam structure (space group No. 72). The conventional unit cell, illustrated in FIG. 1, contains four FeLi₃N₂ formula units (f.u.). The space group allows eleven distinct crystallographic sites; the 4a, 4b, 4c, 4d, and 8e positions are fixed by symmetry, while the 8f, 8g, 8h, 8i, 8j, and 16k sites have variable coordinates and thus can be multiply occupied. In FeLi₃N₂, iron and nitrogen ions occupy the 4a and 8j sites, respectively, and the lithium ions fill the 4b and 8g sites. FeLi₃N₂ contains infinite chains of edge-sharing FeN₄ tetrahedra along the c-direction. These chains are isoelectronic to the SiS₂ one-dimensional macromolecule and form nearly hexagonal arrays linked by sharing common edges with LiN₄ tetrahedra. Two of these tetrahedra are highlighted with speckling in FIG. 1.

Calculation Procedures

Electronic total energies E were computed for the primitive cells (containing two formula units, f.u.) with the Vienna ab initio simulation package (VASP), which implements DFT using a plane wave basis set. Projector-augmented wave potentials were employed for the elemental constituents, and a generalized gradient approximation (GGA) was used for the exchange-correlation energy functional μ_(xc). Non-magnetic calculations were performed for all materials. In addition, spin-polarized calculations were done for some of the Pd-containing compounds to assess the possibility of magnetic states. An interpolation formula was used for the correlation component of μ_(xc) in the spin-polarized cases. For all the XLi₃N₂ and XLi₃N₂H_(n) compounds a plane wave cutoff energy of 900 eV was imposed and (6 6 6) Monkhorst-type k-point grids having 45 points in the irreducible Brillouin zone were employed. In each case at least two simultaneous relaxations of the lattice constants and nuclear coordinates not fixed by the space group were carried out. The electronic total energies and forces were converged to 10⁻⁶ eV/cell and 10⁻⁴ eV/Å, respectively. Calculations for the H₂, N₂ molecules and the elemental metals Li, X were performed with the same computational machinery to the same levels of precision.

Enthalpies of formation, ΔH, were obtained from total energy differences:

ΔH(XLi₃N₂)=E(XLi₃N₂)−E(X)−3E(Li)−E(N₂)   (1)

for the parent compounds, and

ΔH(XLi₃N₂H_(n))=(2/n)[E(XLi₃N₂H_(n))−E(X)−3E(Li)−E(N₂)−(n/2)E(H₂)   (2)

for the hydrides, where n is the number of H atoms in a given configuration. Each ΔH, specified per XLi₃N₂ formula unit (f.u.) in equation (1) and per H₂ molecule in equation (2), is the standard enthalpy of formation at zero temperature in the absence of zero point energy contributions. A negative ΔH indicates stability of the material relative to its elemental solid and molecular constituents.

Results of Calculations

XLi₃N₂ Parent Compounds

Table I lists AH(XLi₃N₂) values calculated according to equation (1) for X═Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Al, Ga, Ge, In, Sn, and Sb.

TABLE I ΔH(XLi₃N₂) Compound (kJ/mole f.u.) YLi₃N₂ −330 ZrLi₃N₂ −337 NbLi₃N₂ −267 MoLi₃N₂ −230 TcLi₃N₂ −200 RuLi₃N₂ −137 RhLi₃N₂ −95 PdLi₃N₂ +20 AgLi₃N₂ +127 CdLi₃N₂ +15 AlLi₃N₂ −482 (−510) GaLi₃N₂ −285 (−317) GeLi₃N₂ −68 InLi₃N₂ −122 SnLi₃N₂ −11 SbLi₃N₂ +76

Except for PdLi₃N₂, AgLi₃N₂, CdLi₃N₂, and SbLi₃N₂, ΔH is negative suggesting that all the other XLi₃N₂ compounds form. This is certainly correct for AlLi₃N₂ and GaLi₃N₂, which are known to exist, but in the cubic |ā3 structure. ΔH calculated for those compounds in the cubic structure is given in parentheses in Table I. These two cases underscore the likely possibility that the actual space group characterizing the other XLi₃N₂ materials in Table I having ΔH<0 may well differ from the Ibam FeLi₃N₂ template on which the calculations are based. In that circumstance the ΔH entry in Table I sets an upper bound on the enthalpy of formation; for AlLi₃N₂ and GaLi₃N₂ it is clear that ΔH for the actual cubic structure is more negative than that calculated assuming the Tbam space group. It is also possible that PdLi₃N₂ and CdLi₃N₂, for which ΔH is positive but relatively small in Table I, form a structure other than Ibam having ΔH<0. That possibility is more distant for AgLi₃N₂ and SbLi₃N₂ since ΔH for each of those compounds is substantially more positive. Spin-polarized calculations for PdLi₃N₂ yielded a negligible magnetic moment and a total energy identical to that from the non-magnetic calculation.

Plots of the electronic density of states (DoS) were calculated for each XLi₃N₂ compound. The DoS is zero at the Fermi level ε_(F) with an energy gap separating the highest occupied and lowest empty states for YLi₃N₂, AlLi₃N₂, GaLi₃N₂, and InLi₃N₂, indicating that those materials are insulators. All the other compounds have a non-zero DoS at ε_(F) and are thus metals.

XLi₃N₂H_(n) Hydrides

Since the fixed-coordinate 4a and 4b sites in the Ibam structure are occupied by X and Li, respectively, the 4c, 4d, 8e, 8f, 8g, 8h, 8i, 8j, and 16k sites, and combinations of them, are available for occupation by hydrogen. All these sites other than the 4c, 4d, and 8c can be multiply occupied, so that in principle an infinite number of hydrogen configurations are possible. For each element X calculations of ΔH(XLi₃N₂H_(n)) as defined by Equation (2) were performed to assess whether stable [ΔH(XLi₃N₂H_(n))<0] hydride configurations exist and to find the most stable configuration, that for which ΔH(XLi₃N₂H_(n)) is a minimum. According to the van't Hoff relation

ln p/p ₀ =ΔH/RT−ΔS/R,   (3)

where ΔS is the entropy of formation and R the gas constant, the configuration having the most negative ΔH is that which is stable at the lowest H₂ pressure p.

Calculations were carried out for hydrogen filling of the 4c, 4d, 8e, 8j, and 16k individual sites and for the 4d8j, 8j₁8j₂, 4d8j₁8j₂, 4d16k, 8j16k, and 16k₁16k₂ combinations. Each of these configurations yielded ΔH<0 for at least some elements X in the 3d transition metals, and the set of choices is likely sufficiently comprehensive to ensure that the configuration having the minimum ΔH is identified for each XLi₃N₂H_(n). The results are compiled in Table II.

There are several negative ΔH values for every X, suggesting the possibility of hydride formation in each case. The minimum ΔH for a given XLi₃N₂H_(n) is highlighted in bold. None of these hydrides is known to exist.

TABLE II ΔH(XLi₃N₂H_(n)) (kJ/mole H₂) X H sites Y Zr Nb Mo Tc Ru Rh Pd Ag Cd Al Ga Ge In Sn Sb  4c 273 204 367 535 611 618 545 568 667 573 115 416 697 709 824 861  4d −352 −324 −129 16 47 125 179 212 268 16 −277 116 237 159 223 296  8e −7 −64 −94 −21 29 136 249 386 562 521 −7 180 75 375 248 225  8j −281 −258 −161 −74 −105 −39 −65 −110 −147 −101 −86 −101 −38 −71 −34 −20 16k −51 −36 −12 −2 −20 −43 −94 −136 −144 −151 127 −151 −151 −144 −147 −160  4d8j −207 −151 −65 −7 58 78 26 35 36 21 −197 47 95 50 −63 52  8j₁8j₂ −223 −202 −151 −115 −88 −65 −58 −19 42 −29 −273 −175 −66 −106 −43 5  4d8j₁8j₂ −127 −93 −45 −13 −12 −2 −18 −31 15 −49 −86 −27 10 −17 3 5  4d16k −15 14 51 76 67 48 −1 −49 −76 −97 −55 −77 −59 −84 −77 −76  8j16k −98 −87 −65 −51 −61 −78 −113 −140 −119 −72 −73 −72 −70 −79 −69 −65 16k₁16k₂ −33 −36 −34 −35 −53 −61 −73 −65 −42 −24 −53 −43 −40 −25 −30 −31

It is clear from Table II that the most stable hydrogen configuration varies with X. The 4d sites provide the greatest stability (i.e., most negative ΔH per mole H₂) for X═Y, Zr, Al; the 8j sites for Nb, Tc, Ag; the 16k sites for Cd, Ge, In, Sn, Sb; the 8j₁8j₂ combination for Mo, Ga; and the 8j16k configuration for Ru, Rh, Pd. Spin-polarized calculations for the PdLi₃N₂H₆ (8j16k) hydride (occupied hydrogen sites in parentheses) produced an insignificant magnetic moment and a total energy identical to the non-magnetic result. RuLi₃N₂H₆ (8j16k), RhLi₃N₂H₆ (8j16k), and PdLi₃N₂H₆ (8j16k) contain the most hydrogen atoms per formula unit of all the hydrides featuring minimum ΔH.

The most stable hydride configuration for X═Y, Zr, Mo, Tc, Ag, and Cd is the same as that for the cognate 3d elements (i.e., those one row above in the periodic table) As specific examples, the 4d hydrogen sites lead to the greatest stability in YLi₃N₂H and ScLi₃N₂H, as do the 8j₁8j₂ sites in both MoLi₃N₂H₄ and CrLi₃N₂H₄. In contrast, NbLi₃N₂ (8j), RuLi₃N₂H₆ (8j16k), RhLi₃N₂H₆ (8j16k), and PdLi₃N₂H₆ (8j16k) are found to be most stable here, while their 3d analogs are VLi₃N₂H₄ (8j₁18j₂), FeLi₃N₂H₂ (8j), CoLi₃N₂H₂ (8j), and NiLi₃N₂H₂ (8j), respectively.

Table III summarizes the minimum ΔH results from Table II and includes the hydrogen mass percentage for each hypothetical hydride.

TABLE III XLi₃N₂H_(n) hydride (H configuration in ΔH(XLi₃N₂H_(n)) ΔH * (XLi₃N₂H_(n)) conventional cell) (kJ/mole H₂) (kJ/mole H₂) mass % H YLi₃N₂H (4d) −352 +308 0.7 ZrLi₃N₂H (4d) −324 +350 0.7 NbLi₃N₂H₂ (8j) −161 +106 1.4 MoLi₃N₂H₄ (8j₁8j₂) −115 −0.2 2.7 TcLi₃N₂H₂ (8j) −105 +95 1.4 RuLi₃N₂H₆ (8j16k) −78 −32 3.9 RhLi₃N₂H₆ (8j16k) −113 −81 3.8 PdLi₃N₂H₆ (8j16k) −140 −147 3.7 AgLi₃N₂H₂ (8j) −147 −274 1.3 CdLi₃N₂H₄ (16k) −151 −158 2.4 AlLi₃N₂H (4d) −277 +743 1.3 GaLi₃N₂H₄ (8j₁8j₂) −175 −17 3.3 GeLi₃N₂H₄ (16k) −151 −117 3.2 InLi₃N₂H₄ (16k) −144 −83 2.4 SnLi₃N₂H₄ (16k) −147 −142 2.3 SbLi₃N₂H₄ (16k) −160 −198 2.3

If any of these were to form in a different crystal structure, or with an alternate stoichiometry, the hydrogen content could certainly change.

The formation enthalpy for the hydride with respect to its parent compound,

ΔH*(XLi₃N₂H_(n))≡(2/n)[E _(el)(XLi₃N₂H_(n))−E _(el)(XLi₃N₂)−(n/2)E _(el)(H₂)],   (4)

is also given in Table III. For MoLi₃N₂H₄ (8j₁8j₂), RuLi₃N₂H₆ (8j16k), RhLi₃N₂H₆ (8j16k), GaLi₃N₂H₄ (8j₁8j₂), GeLi₃N₂H₄ (16k), InLi₃N₂H₄ (16k), and SnLi₃N₂H₄ (16k), ΔH*(XLi₃N₂H_(n)) and ΔH(XLi₃N₂) in Table I are both negative. That is, the hydride is stable with respect to a stable parent, suggesting the possibility of cycling between the two, a situation much more desirable from an applications perspective than cycling between the XLi₃N₂H_(n) hydride and its four elemental constituents (X and Li metals, H₂ and N₂ gases). It is also significant to observe that for all the stable parents in Table I there are XLi₃N₂H, hydrides for which ΔH*(XLi₃N₂H_(n)) is negative, including X═Y. Zr, Nb, Tc, and Al in Table III for which the hydride entry there is characterized by ΔH*>0.

To illustrate the effect of hydriding on the electronic structure, the DoS for the hydrides characterized by the minimum ΔH (Table III) were calculated. For most of the hydrides hydrogen-derived bands appear below the bottom of the valence bands of the parent, similar to the behavior of LaNi₅ on hydriding. From the DoS at the Fermi energy ε_(F) it was apparent that the YLi₃N₂H (4d), ZrLi₃N₂H (4d), NbLi₃N₂H₂ (8j), MoLi₃N₂H₄ (8j₁8j₂), RuLi₃N₂H₆ (8j16k), RhLi₃N₂H₆ (8j16k), PdLi₃N₂H₆ (8j16k), CdLi₃N₂H₄ (16k) and AlLi₃N₂H (4d), GeLi₃N₂H₄ (16k), InLi₃N₂H₄ (16k), SnLi₃N₂H₄ (16k) and SbLi₃N₂H₄ (16k) hydrides are metals. TcLi₃N₂H₂ (8j), AgLi₃N₂H₂ (8j), and GaLi₃N₂H₄ (8j₁8j₂) are all insulators (zero DoS and gaps at ε_(F)).

Most of the XLi₃N₂ parent compounds are metals and remain metallic on hydrogen uptake. While TcLi₃N₂ and AgLi₃N₂ are metals, their hydrides are insulators. On the other hand, YLi₃N₂, AlLi₃N₂, and InLi₃N₂ are insulating, but their hydrides are predicted to be metallic. Were these materials to form, such metal

insulator transitions might be exploited for hydrogen sensor applications.

These state-of-the-art electronic structure calculations demonstrate the likelihood of the availability of YLi₃N₂, ZrLi₃N₂, NbLi₃N₂, MoLi₃N₂, TcLi₃N₂, RuLi₃N₂, RhLi₃N₂, GeLi₃N₂, InLi₃N₂, and SnLi₃N₂. These parent compounds feature large, negative enthalpies of formation as illustrated in Table I. It is also important to observe that the absence of a stable parent compound, as may be the case for X═Pd, Ag, Cd, and Sb in Table I, does not necessarily preclude the existence of a corresponding hydride. Many such systems are known, among them NaAlH₄ and Mg₂FeH₆, whose antecedents NaAl and Mg₂Fe do not form.

The electronic structure calculations also indicate that all considered hydrides have substantial negative enthalpies of formation as illustrated in Tables II and III. These hydrides include YLi₃N₂H_(n), ZrLi₃N₂H_(n), NbLi₃N₂H_(n), MoLi₃N₂H_(n), TcLi₃N₂H_(n), RuLi₃N₂H_(n), RhLi₃N₂H_(n), PdLi₃N₂H_(n), AgLi₃N₂H_(n), CdLi₃N₂H_(n), AlLi₃N₂H_(n), GaLi₃N₂H_(n), GeLi₃N₂H_(n), InLi₃N₂H_(n), SnLi₃N₂H_(n), and SbLi₃N₂H_(n). Here n is an integer in the range of 1-6.

It is also noted (as illustrated in Table III) that the hydrides MoLi₃N₂H₄ (8j₁8j₂), RuLi₃N₂H₆ (8j16k), RhLi₃N₂H₆ (8j16k), GaLi₃N₂H₄ (8j₁8j₂), GeLi₃N₂H₄ (16k), InLi₃N₂H₄ (16k), and SnLi₃N₂H₄ (16k) are characterized by minimum ΔH with respect to the elemental constituents as well as negative values of ΔH* with respect to the parent materials. The electronic structure calculations indicate that these hydrides are thermodynamically stable with respect to their respective stable parent compounds (i.e., those having ΔH<0 in Table I). This relationship suggests that hydrogen may be cyclically absorbed and released using these hydrides and their parent compounds.

The above described synthesis of FeLi₃N₂, adapted for the properties of the specific properties of the elements of the 4d transition metal group and of germanium, indium, and tin, provides a basis for the synthesis of the above listed parent compounds. The hydrides might be prepared by reacting appropriate amounts of LiNH₂, LiH, and the elemental metals X, or similar schemes with the XN nitrides and Li₃N. 

1. Any one or more of the ternary nitrides of the following compositional formulas: YLi₃N₂, ZrLi₃N₂, NbLi₃N₂, MoLi₃N₂, TcLi₃N₂, RuLi₃N₂, RhLi₃N₂, GeLi₃N₂, InLi₃N₂, and SnLi₃N₂.
 2. A hydrogen storage composition consisting essentially of any one or more of the ternary nitrides as recited in claim 1 for the storage of hydrogen as a hydride of the ternary nitride.
 3. Any one or more of the hydrides of ternary nitrides having the following compositional formulas: YLi₃N₂H_(n), ZrLi₃N₂H_(n), NbLi₃N₂H_(n), MoLi₃N₂H_(n), TcLi₃N₂H_(n), RuLi₃N₂H_(n), RhLi₃N₂H_(n), PdLi₃N₂H_(n), AgLi₃N₂H_(n), CdLi₃N₂H_(n), AlLi₃N₂H_(n), GaLi₃N₂H_(n), GeLi₃N₂H_(n), InLi₃N₂H_(n), SnLi₃N₂H_(n), and SbLi₃N₂H_(n), where n is an integer having a value of from 1-6.
 4. Any one or more of the hydrides as recited in claim 3 having the following structural formulas: YLi₃N₂H, ZrLi₃N₂H, NbLi₃N₂H₂, MoLi₃N₂H₄, TcLi₃N₂H₂, RuLi₃N₂H₆, RhLi₃N₂H₆, PdLi₃N₂H₆, AgLi₃N₂H₂, CdLi₃N₂H₄, AlLi₃N₂H, GaLi₃N₂H₄, GeLi₃N₂H₄, InLi₃N₂H₄, SnLi₃N₂H₄, and SbLi₃N₂H₄.
 5. A hydrogen storage composition consisting essentially of any one or more of the hydrides as recited in claim
 3. 6. A hydrogen storage composition consisting essentially of any one or more of the hydrides as recited in claim
 4. 